Machines such as wheel loaders, backhoes, fork lifts, and other types of heavy equipment use multiple actuators supplied with hydraulic fluid from one or more pumps on the machine to accomplish a variety of tasks. Movement of these actuators is typically controlled based on an actuation position of an operator interface device. For example, when a machine operator pulls a joystick controller rearward or pushes the joystick controller forward, one or more lift actuators mounted on the machine either extend to lift a work tool away from a ground surface or retract to lower the work tool back toward the ground surface. Similarly, when the operator pushes the same or another joystick controller to the left or right, tilt actuators mounted on the wheel loader either extend to dump the work tool downward toward the ground surface or retract to rack the work tool backward away from the work surface. The forces generated by the lift and tilt actuators are related to hydraulic surface areas within each of the actuators and a pressure of fluid supplied to the actuators.
The pressure of the fluid supplied to the actuators is generally limited by one or more pressure relief valves to avoid damage to system components. Each pressure relief valve can be situated, for example, between a control valve and a corresponding actuator, and configured to selectively open and relieve actuator pressures when the pressures reach or exceed a particular level. Historically, pressure relief valves have been hydro-mechanical components that are spring-biased and configured to move between two positions based on actuator pressures, including a flow-passing position at which actuator pressure is relieved, and a flow-blocking position at which actuator pressure is allowed to build. The pressure threshold at which the conventional pressure relief valve is moved to the flow-passing position is dependent upon a factory-set spring bias (i.e., a threshold setting), and remains unchanged during operation of the machine throughout the machine's life.
Although successful at helping to avoid damage to system components in some situations, pressure relief valves of the type described above may still be less than optimal. In particular, a pressure relief valve that has only a single pressure setting may not provide all the functionality required to fully protect system components and/or loads carried by the machine. For example, during movement of an actuator under maximum system pressure (i.e., a pressure just less than the pressure required to open the relief valve), the actuator can be damaged when a travel end-stop of the actuator is reached (i.e., when the actuator hits the end-stop during travel at full force).
An alternative type of pressure relief valve is disclosed in U.S. Pat. No. 3,937,128 that issued to Hicks et al. on Feb. 10, 1976 (the '128 patent). In particular, the '128 patent describes a dual stage pressure relief valve for use with a hydraulic jack. The relief valve is configured to establish a relatively low operating pressure for normally actuating the hydraulic jack, and to selectively increase the system pressure during a selected operation. The relief valve is switched between low- and high-pressure settings based on movement of the hydraulic jack near an end-of-stroke position.
While the dual pressure settings of the '128 patent may increase the functionality of the pressure relief valve, it may lack applicability. In particular, the step-change in pressure levels may be problematic in heavy-loading situations, where a sudden shift from high-pressure to low-pressure could cause load instabilities. In addition, the hydraulic system of the '128 patent still does not address damage that can occur at an end-of-stroke position during actuator movement at high-pressure.
The present disclosure is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.